Process and apparatus for conducting electrophoresis and transfer

ABSTRACT

An apparatus for carrying out horizontal gel electrophoresis for separation and subsequent vacuum-assisted transportation of macromolecules to a support membrane to facilitate detection. The entire procedure is conducted in one cartridge. A method for conducting electrophoresis and subsequent vacuum-assisted transfer using the apparatus of the present invention is also disclosed.

FIELD OF THE INVENTION

This invention relates to processes and apparatus for carrying outhorizontal gel electrophoresis for separation and subsequent vacuumassisted transportation of macromolecules to a support membrane tofacilitate detection.

BACKGROUND OF THE INVENTION

The process known as electrophoresis involves the migration of chargedmolecules through a suitable retarding medium under the influence of anelectric field. Generally, the compounds of higher molecular weightmigrate at a slower rate through the medium than do the compounds oflower molecular weight. Devices have been provided previously forcarrying out electrophoresis. An example of such a device is U.S. Pat.No. 4,415,418 in which a tray is provided with a raised platform at thecenter. Removable partitions are placed in the tray at opposite ends ofthe platform, and a conventional electrophoresis gel is poured over theplatform to form a thin layer. When the gel has cooled, the partitionsare removed. A comb is provided to form wells across the surface of thegel. Substances that are to be subjected to electrophoresis aredelivered into each of the wells, and the tray is at least partiallyfilled with an electrolyte buffer. Electrodes are positioned at each endof the tray and a sufficient voltage difference is applied to theelectrodes to cause migration of the molecules of the substance in thewells across the length of the gel, separated according to theirmolecular weight. After electrophoresis, the gel is removed from theoriginal casting tray, and placed in a dish containing depurinationsolution. Approximately thirty minutes later this solution is poured outby tipping the dish toward one edge while the gel is held with thefingers. It is important to use great care during this procedure toprevent the gel from breaking because there is no gel support structureand subsequent processing is possible only with an integral gel. Adenaturation solution is then added to the dish and incubation iscontinued for approximately thirty minutes. Again, the solution iscarefully poured off. Then neutralization buffer is added and incubationis continued for thirty additional minutes.

In accordance with conventional techniques, transfer of the nucleicacids is accomplished by placing a piece of filter paper, which is aswide as and longer than the gel, on a platform which is suspended abovea solution of 10× saturated saline citrate buffer (SSC). The ends of thefilter paper are long enough to hang off the ends of the platform anddip into the 10× SSC. Thus, the filter paper acts as a wick to absorbthe SSC solution. The gel is removed from the dish and placed on top ofthe filter paper saturated with 10× SSC. Next, a piece of membranefilter paper which is the same size as the gel is saturated with 10× SSCand placed on top of the gel. The nucleic acids are eventually bound tothe membrane filter paper. Another piece of saturated filter paper, thesame size as the gel, is placed on top of the membrane. The entirelayered unit is then smoothed to remove any air bubbles that may existbetween the gel and the filter paper. Finally, a stack of paper towels,the same size as the gel, is positioned on top of the layered unit.

Over a period of about 12 to 16 hours, the 10× SSC solution is drawn upthrough the gel by capillary action and the nucleic acids aretransferred out of the gel into the membrane above. The paper towelsabsorb the excess buffer and provide the force for capillary action. Atthe end of the transfer period, the entire layered unit is disassembledand the membrane is removed for hybridization. This technique isdescribed in an article Southern, E., "Detection of Specific SequencesAmong DNA Fragments Separated by Gel Electrophoresis," . J. Mol. Biol.,98:503 (1975).

Although the trays such as the one described in U.S. Pat. No. 4,415,418are convenient for carrying out electrophoresis, they are not suitablefor situations where a large number of samples must be tested in arelatively short period of time.

Therefore, the prior art uses a tedious multi-step, multi-apparatusprocess for preparing nucleic acid fragments for subsequenthybridization. Four steps are generally undertaken to achievepreparation of the sample for hybridization. Electrophoresis waspreviously described. Depurination removes purine bases from nucleicacids. Denaturation involves separating the strands of nucleic acids andbreaks down the depurinated nucleic acids into suitable size to alloweventual transfer of the fragments out of the gel. Transfer involvesallowing the fragments to go out of the gel onto the porous membrane.

OBJECTS OF THE INVENTION

It is the principal object of the present invention to provide oneapparatus wherein the steps of electrophoresis, depurination,denaturation and transfer to a membrane may all be carried out.

It is an object of the present invention to standardize and simplify theelectrophoresis and transfer techniques to facilitate applications ofmolecular biology.

It is a further object of the present invention to develop an apparatusfor electrophoresis and transfer which offers considerable time and costsavings.

Another object of the present invention is to avoid the difficultiesgenerally associated with prior art methods and apparatus with respectto handling the gel and further processing.

SUMMARY OF THE INVENTION

In accordance with the present invention, nucleic acid fragments such asDNA and RNA may be prepared for subsequent hybridization using theprocess and apparatus of this invention. The process and apparatus areparticularly useful for detection of gene rearrangements, restrictionfragment polymorphisms and restriction fragment patterns. The processand apparatus enables rapid screening of tissue specimens and bodyfluids for the presence of infectious viruses such as Human Papillomavirus, for typing B-cell and T-cell monoclonal populations, and forscreening patients for the development of cancer or other diseasestates.

Thus, the apparatus of the present invention combines in a single unitmeans for conducting electrophoresis and transfer, and includes a trayhaving opposite side walls, opposite end walls and a bottom wall. Thereis a central platform in the tray, with a vacuum chamber between theplatform and the bottom wall. The platform surface is pervious toliquid. A liquid reservoir is provided adjacent each of the end wallsand electrophoresis electrodes are mounted in the reservoirs. A conduitis provided for transferring liquid into and out of the tray. The trayis covered by a lid. The apparatus is adapted to perform electrophoresisand transfer without removing the gel from the tray.

The process of the present invention is performed by placing a transfermembrane on a porous platform in a tray. The gel is placed on themembrane. Samples are deposited in spaced wells in the gel. Anelectrophoresis buffer is pumped into the tray to cover the gel and theelectrodes and an electric potential is applied between the electrodes.After the electrophoresis step, depurination and denaturation arecarried out while the gel remains in the tray. During the denaturationstep, the liquid is drawn through the porous membrane to cause thedisplaced samples to be transferred from the gel to the membrane.

DETAILED DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is illustrated in theaccompanying drawings, in which:

FIG. 1 is a top plan view of the electrophoresis transfer tray inaccordance with this invention, with the lid removed;

FIG. 2 is a side elevational view of the cartridge of this invention,partially in cross-section;

FIG. 3 is a detail top plan view of the tray with the perforated plateremoved;

FIG. 4 is a cross-sectional view of the tray along the line 4--4 in FIG.3;

FIG. 5 is a detail view of the plate on which the membrane and gel aresuperimposed;

FIG. 6 is a top plan view of the lid for the cartridge;

FIG. 7 is a cross-sectional view of the lid along the line 7--7 in FIG.6;

FIG. 8 is a cross-sectional view of the lid along the line 8--8 in FIG.6;

FIG. 9 is a detail plan view of the tray with the bridge installed;

FIG. 10 is a front elevational view of the tray with the bridgeinstalled as in FIG. 9; and

FIG. 11 is a cross-sectional view of the bridge and tray along the line11--11 in FIG. 9.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, a cartridge 2 is used for carrying out theelectrophoresis and transfer processes of this invention. The cartridge2 includes a tray 4 and a lid 6. Tray 4 is preferably composed ofpolyvinyl chloride or Delrin, a resin which is a registered trademark ofE. I. duPont de Nemours & Co. Inc.; however, any material which has gooddimensional stability for fabrication and non-electrically conductiveand which is chemically compatible with the intended use is suitable.The tray 4 has opposite side walls 8, end walls 10 and a bottom wall 12.A support surface 14 is provided at the center of the tray. The bottomwall 12 adjacent the end walls 10 slopes downwardly, as shown in FIG. 2away from the support surface 14. A plurality of webs 16 extend inwardlyfrom the end walls 10. The webs facilitate insertion of the gel withouttearing. The webs also align the gel thereby assuring proper orientationof the gel in the electric field.

As shown in FIGS. 2 and 4, a transverse passage 18 is provided in thebottom wall 12 and ports 20 communicate with the passage 18 to allow thecirculation of fluid into and out of the tray. A tubing fitting 22 isprovided on the side wall 8. An identical passage 18 is provided at theopposite end of the tray (FIG. 1) and has corresponding ports 20 and atube fitting 22.

Electrodes are provided at each end of the tray. The electrode 24 is inthe form of a thin wire, preferably of platinum, which is wrapped on aninsulated rod 26. The rod is supported in aligned holes in the webs 16.The electrode 24 extends through the side wall 8 and is connected withan electrical contact element 28 which is adapted to be connected to asource of electric potential.

The support surface 14 is formed of a porous plate 30 (FIG. 11). Anyporous plate is suitable, however, a plate made of a porous polyethyleneis preferred. The porous plate should be about 30% to 85% open tosufficiently pull water. As shown in FIG. 5, the plate 30 has parallelscore lines on each side which are perpendicular, so that small holesthrough the plate are formed at the inner section of the score lines. Ofcourse, any suitable porous plate could be substituted for the plate 30.A porous membrane 31 rests on the plate 30. Membrane 31 must haveproperties such that it is optimum for binding of vacuum assistedtransported DNA fragments. Membrane 31 is preferably comprised of nylonor nitrocellulose such as a nylon membrane comprised of about 0.2 to 1.2micron porosity. However, any fluid-permeable membrane which issufficient to bind nucleic acids would be suitable.

A gel is cast on a separate tray. The gel can be agarose,polyacrylamide, mixed agarose/polyacrylamide or any other materialsuitable for the separation of macromolecules in an electric field.Generally, 0.7% w/v agarose is used. The gel 32 is then placed on thesurface of the membrane 31 in a suitable mold and then transferred tothe tray 4 where it is superimposed on the plate 30, as shown in FIG. 5.A vacuum chamber is provided beneath the plate 30, as shown in FIGS. 2and 3. The chamber includes a pair of ridges 34 extending outwardly froma central channel 36. A tubing fitting 38 extends through the side wall8 and communicates with the channel 36. The fitting 38 is adapted to beconnected by tubing to a suitable vacuum pump. The opposite ends of thegel 32, when placed in the tray 4, abut the ends of the webs 16, asshown in FIG. 11. The side edges of the gel 32 are positioned byengagement with the side walls 8 of the tray. The membrane 31 is spacedinwardly from the edge of the plate 30 along all four sides of theplate, so that the vacuum that is applied beneath the plate will tend tohold the ends and sides of the gel against the plate 30. If additionalmeans are required to keep the gel 32 from floating in the fluid, a bandor tube 39 (FIGS. 9 and 11) may be applied across the surface of the gelat each end. The ends of the tube 39 are secured in the side walls 8.

As shown in FIG. 6, the lid 6 has a shoulder 40 which extends around theperimeter of the lid and engages the inside surface of the side walls 8and the end walls 10. A sealing gasket 42 is retained within a groove inthe shoulder 40. The gasket 42 prevents the leakage of fluid from theinterior of the cartridge. A gas bubble channel 44 is formed in theinterior of the lid 6 and extends along the end walls and side walls ofthe tray. The channel portion at the right side of FIG. 6 is moreshallow than the groove at the left side of FIG. 6, and the groovesextending along the side walls 8 progressively increase in depth fromthe right end to the left end as viewed in FIG. 6. This arrangement ofthe grooves causes the gas bubbles to migrate progressively toward anoutlet port 46 through the lid 6. The outlet port has a tube fitting 48through which the gas bubbles can escape.

The central portion of the lid 6 has a plurality of grooves 50 formed inthe top side of the lid. As shown in FIGS. 6 and 8, ports 52 extend fromthe bottom of the grooves to the lower surface of the lid so that fluidcan pass from the grooves 50 into the interior of the cartridge when thelid is in place. A cover plate 54 encloses the grooves 50 and a tubefitting 56 allows liquid to be conducted through the plate 54 into theinterior of the grooves 50.

In order to enable samples to be delivered into the wells formed in thegel layer 32, a bridge 58 is provided. The bridge aids in directing theoperator to the correct well for sample filling thus making it easier tofill the well. The bridge is received in vertical slots 60 in each ofthe side walls 80 at a position that is aligned with the wells that aremolded in the gel layer. A black strip may be positioned near the wellsto aid in visualizing the wells during sample loading. The bridge 58includes funnel-shaped passages 62 in a shape to receive the tip of apipette for delivering the samples into the wells formed in the gel. Asshown in FIG. 10, arches 64 are formed in the bridge between thepassages 62 to allow fluid circulation between opposite sides of thebridge. If additional recirculation is necessary, the bridge can beremoved prior to electrophoresis.

In operation, a gel layer is formed in accordance with conventionalpractices of a proper size and shape to fit within the tray 4, so thatthe ends abut the ends of the webs 16 and the sides of the gel fitbetween the side walls 8 of the tray. Preferably, the gel is cast in aseparate tray and has a series of wells molded in the gel layer adjacentone end. The gel 32 is then removed from the casting tray and placed onthe flexible permeable membrane 31, so that it occupies the positionshown in FIGS. 5, 9 and 11. The sloping top edge of each web 16 aids inguiding the gel into position in the tray 4. A source of electricpotential is connected with the electrical connectors 28 and a systemfor circulating fluid is connected with the tubing fittings 22. Anelectrophoresis buffer is added to the tray to a depth that fully coversthe electrodes 24 and the gel. The bridge 58 is installed in the slots60 after the gel layer is positioned on the support surface 14. Thesamples are then delivered into the passages 62 from which they passinto the individual wells. The bridge remains in place when the lid 6 isapplied. The electrophoresis buffer is recirculated through the passages18 by the use of a conventional pump, to provide a fluid current passingover the gel. At the end of electrophoresis, the electrophoresis bufferis then pumped out of the tray and a depurination buffer is pumped intothe tray. After a period of time, the depurination buffer is then pumpedout of the tray and a denaturation buffer is pumped into the tray. Aftera suitable elapse of time, the denaturization buffer is withdrawnthrough the vacuum fitting 38 at a relatively slow rate, which transfersthe displaced samples from the gel onto the permeable membrane. As analternative, water or denaturation liquid may be sprayed onto thesurface of the gel through the tube fitting 56 as the liquid is beingwithdrawn through the tube fitting 38. The lid 6 is then removed and thepermeable membrane 31 may be removed from the tray for furtherprocessing. The membrane now contains the displaced samples inpreparation for hybridization.

The cartridge of this invention has the important advantage that itallows electrophoresis and transfer to occur without having to removethe gel from the tray. At the completion of the operation, the nylonmembrane bearing the samples can readily be removed for subsequenttreatment.

The electrophoresis apparatus is designed to accommodate either a largenumber of analytical samples or milligram quantities of fragments forpreparative runs. Typically, the number of samples which may beintroduced into the cartridge may range from about 5 to 15. Generally,10 samples plus two controls has been found to be suitable.

The tray 4 and the lid 6 must be compatible with standardelectrophoresis and nucleic acid transfer reagents. Typical reagentsinclude up to 3 molar (M) salts, acetic acid, 1 M hydrochloric acid and0.5 M sodium hydroxide. Many polymers could be suitable for the presentinvention. Furthermore, in view of the direct current being used duringelectrophoresis, the tray 4 and lid 6 should not conduct electricity.The lid 6 is preferably plexiglass (acrylic) since a further advantageis obtained with the use of a clear cover since it would allow visualtracking of optional dyes during electrophoresis.

Using one buffer or solution throughout the foregoing procedure is moreefficient and economical than using a different buffer or solutionduring each of the electrophoresis, depurination, denaturation andtransfer stages. Alternatively, four separate solutions may be used inthe practice of the present invention. For example, duringelectrophoresis any buffer well-known in the art is suitable such as anysolution of a weak acid or base and its salts, such as acetates,borates, phosphates and phthalates, which behave as buffers. Typicalcompounds used in preparing buffers include acetic acid, phenylaceticacid, sodium acetate, ethylene diamine tetraacetic acid (EDTA),phosphoric acid, boric acid, hydrochloric acid, sodium hydroxide, sodiumchloride and the like. During electrophoresis, a buffer comprised of 40mM tris-acetate, pH 8, 12 mM sodium acetate and 2 mM EDTA, pH 8, ispreferred.

During depurination, any solution which chemically assists indepurination or depyrimidination would be useful. These solutions arewell-known in the art. A buffer comprised of 0.25 M hydrochloric acid ispreferred.

During denaturation, any solution which assists in breaking the hydrogenbonds between the nucleic acid strands is suitable. These solutions arealso well-known in the art. For example, water and heat may providesatisfactory results, also formamide or any alkali such as sodiumhydroxide or potassium hydroxide. In the present invention a solution of0.5 M sodium hydroxide is preferred.

As the transfer solution, any solution which allows transfer and bindingof the nucleic acid strands to the membrane would be suitable. Thesesolutions are also well-known in the art. Advantageously, in thepractice of the present invention the transfer solution is preferably0.5 M sodium hydroxide, the same as the denaturation solution.

The time periods used in each of the above-described stages may varyover a wide range depending on the processing conditions. For instance,each of electrophoresis, denaturation and depurination may require fromabout 10 minutes to 5 hours. Useful techniques to decrease the amount oftime required for processing include increasing the voltage, usinglarger ports, selecting the optimum thickness of the gel, differentsizes of membranes and the support plate, and the like. A particularadvantage of the present process is that the time required forelectrophoresis is approximately 40-65% of the typical time required forelectrophoresis using prior art apparatus and techniques. A significanttime savings is offered by the present invention because of thecirculation of fluids, i.e., buffers, through the electrophoresistransfer cartridge, thereby allowing a constant pH and temperature to bemaintained during the process. In addition, the geometry of thecartridge results in concentrating the electric field within the gelwhich also speeds the process.

Furthermore, by maintaining a relatively frequent circulation of thebuffer, a smaller volume of buffer is suitable in contrast to the bufferrequirements of prior art electrophoresis processes.

The transfer step may require from about 10 minutes to two hours.Generally, about 60 minutes produces adequate results. This step alsooffers a considerable time savings over prior art transfer techniques.Capillary transfer, for instance, requires about 12 hours, squash blottransfer requires about 3 hours and standard electrotransfer requiresabout four hours.

A further advantage of the present invention is the time and laborsavings that results since the apparatus is a cartridge. The decrease inthe number of mechanical steps to be performed by a technician oroperator assists in maintaining the accuracy of the procedure since adecrease in the number of necessary steps to be performed also minimizeserror.

Other components are useful to achieve the objects of the presentinvention in addition to the above-described electrophoresis transfercartridge. For instance, a microprocessor controller may be used toautomate electrophoresis and DNA/RNA transfer. Such a controller wouldcontrol the voltage and time for electrophoresis, the valves forreagents, the pumps to add and remove reagents and the vacuum system forDNA transfer. The electrophoresis transfer cartridge may be connected toa variety of standard laboratory equipment including peristaltic pumps.

The following example is intended to demonstrate one method that may beused to practice the present invention. The following is not intended tolimit the invention in any way.

EXAMPLE Electrophoresis Transfer Process

A 14 cm long×11 cm wide×0.65 cm deep 0.7% w/v agarose gel having a nylonmembrane on its bottom surface was placed and aligned in a polyvinylchloride electrophoresis transfer tray having inside dimensions of 20 cmlong×11 cm wide×2 cm deep. A liquid pervious platform or support platemade of porous polyethylene rests on the inside of the tray.Approximately 200 ml of an electrophoresis buffer comprised of 40 mMtris-acetate, 12 mM sodium acetate and 2 mM EDTA was added by hand.Restriction enzyme digested human genomic DNA samples were loaded intothe wells and a plexiglass top was placed over the polyvinyl chloridetray. The unit was then plugged into a power supply which was set at 90volts. The power was turned on. Electrophoresis was allowed to continuefor approximately ten minutes. Then the buffer was recirculated at arate of approximately 25 ml/min for the duration of electrophoresis.Electrophoresis continued for approximately two hours. The power wasturned off and the electrophoresis buffer was pumped out.

Approximately 200 ml of a depurination solution comprising 0.25 Mhydrochloric acid was pumped into the polyvinyl chloride tray. Thesolution was allowed to stand for approximately 15 minutes and then thesolution was pumped out. Next, approximately 200 ml of a denaturationsolution was added to the polyvinyl chloride tray. A vacuum pump wasturned on to withdraw fluid through the porous polyethylene plate atabout 2 ml/min. This was continued for 60 minutes to allow the DNA totransfer from the gel to the nylon membrane below the gel. The unit wasturned off.

While this invention has been illustrated and described in accordancewith a preferred embodiment, it is recognized that variations andchanges may be made therein without departing from the invention as setforth in the claims.

What is claimed is:
 1. Apparatus for conducting electrophoresis andsubsequent transfer, comprising:a tray having opposite side walls,opposite end walls and a bottom wall; platform means in said tray forsupporting a gel, said platform means including a support plate that ispervious to liquid; said tray and platform means defining a chamberbetween said plate and said bottom wall; electrode means for applying anelectric field in said tray between said end walls, said platform meansbeing between said electrode means; liquid reservoir means adjacent saidend walls; conduit means for transferring liquid into and out of saidtray; and lid means for covering said tray.
 2. The apparatus accordingto claim 1 wherein said liquid reservoir means extends between said sidewalls and along each end wall and said bottom wall, said conduit meansincluding passages in said tray communicating with said reservoir meanswhereby liquid is transferred into and out of said tray through saidreservoir means.
 3. The apparatus according to claim 2 wherein saidliquid reservoir means includes a plurality of webs extending inwardlyfrom said end wall toward said platform means, and said electrode meansincludes an elongated electrode extending through and supported by saidwebs.
 4. The apparatus according to claim 1 including means for applyinga vacuum to said chamber, whereby liquid passes through said plate intosaid chamber upon the application of a vacuum in said chamber.
 5. Theapparatus according to claim 4 wherein said apparatus includes a porousmembrane on said plate, whereby a gel may be superimposed on saidmembrane and said plate for electrophoresis and transfer of samples fromthe gel to the membrane.
 6. The apparatus according to claim 4 whereinsaid liquid reservoir means extends between said side walls and alongeach end wall and said bottom wall and includes a plurality of websextending inwardly from said end wall toward said pervious plate, saidwebs terminating at a predetermined distance from said plate, wherebysaid webs serve as guides for locating the ends of the gel when placedin said tray nf said plate.
 7. The apparatus according to claim 1wherein said lid means includes a lid extending across said tray andbeing supported by said side walls and said end walls, said lid meansincluding means for supplying liquid to the interior of said tray andincluding guide means for guiding vapor bubbles from the interior ofsaid tray through a outlet opening in said lid.
 8. The apparatusaccording to claim 1 including bridge means and means mounting saidbridge in said tray between said side walls, said bridge means includinga plurality of passages for receiving samples in position to deposit thesamples in wells formed in a gel between said bridge means and saidplatform means.
 9. The apparatus according to claim 1 includingrestraining means for holding a gel in said tray in position on saidplatform means during electrophoresis and transfer.
 10. A method ofconducting electrophoresis and subsequent transfer, comprising:placing aliquid pervious membrane on a liquid pervious platform in a tray;placing a gel in said tray on said membrane and said platform; insertingsamples in wells formed in the gel; covering the gel with anelectrophoresis buffer; applying an electric potential through the gelto cause migration of samples in response to the electric potential;subjecting the gel to depurination solution in the tray; conducting adenaturation transfer solution through said tray in contact with saidgel; applying a vacuum under said liquid pervious platform, therebytransferring the samples from the gel to the membrane by means of thedenaturation transfer solution; and subsequently removing the membranefrom the tray.
 11. The method according to claim 10 wherein said samplescomprise nucleic acids.
 12. The method according to claim 10 comprisingthe additional step of removing said electrophoresis buffer afterapplying said electric potential.
 13. The method according to claim 12wherein said electrophoresis buffer is comprised of about 40 mMtris-acetate, pH 8; about 12 mM sodium acetate and about 2 mM EDTA, pH8.
 14. The method according to claim 12 comprising the additional stepof removing said depurination solution prior to conducting saiddenaturation transfer solution through said tray.
 15. The methodaccording to claim 14 wherein said depurination solution is comprised ofabout 0.25 M hydrochloric acid.
 16. The method according to claim 12wherein said denaturation transfer solution is comprised of about 0.5 Msodium hydroxide.
 17. The method according to claim 10 wherein said gelis an agarose gel.
 18. The method according to claim 10 wherein saidmembrane is comprised of porous nylon.
 19. The method according to claim10 wherein said liquid pervious platform is comprised of porouspolyethylene.
 20. Apparatus for transferring nucleic acid fragments froma gel to a support membrane, comprising:a tray having a bottom wall;platform means in said tray, said platform means including a poroussupport plate; said tray and platform means defining a chamber betweensaid plate and said bottom wall; liquid reservoir means above saidplate, said reservoir means comprising a wall extending above andsurrounding the perimeter of said support plate and arranged to containliquid above said support plate; and a fluid port communicating withsaid chamber for withdrawing fluid.
 21. The apparatus according to claim20 wherein said platform means includes guides for positioning the gelwhen placed on said plate over the membrane.
 22. The apparatus accordingto claim 20 including means for drawing fluid from said chamber meansand thereby drawing a vacuum in said chamber means.
 23. The apparatusaccording to claim 20 including electrode means in said tray at oppositesides of said reservoir means for conducting electrophoresis in a gelplaced on said plate.
 24. A method of conducting the transfer of nucleicacid fragments from a gel to a support membrane, comprising:placing aliquid pervious membrane on a liquid perivous platform in a tray;placing a gel in said tray on said membrane and said platform; placing atransfer solution in the tray in contact with the gel; applying a vacuumunder said liquid pervious platform, thereby transferring the samplesfrom the gel to the membrane by means of the transfer solution, andsubsequently removing the membrane from the tray.
 25. The methodaccording to claim 24 wherein the membrane is formed of porous nylon.26. The method according to claim 24 wherein the platform is formed ofporous polyethylene.